KEYS: trusted: explicitly use tpm_chip structure from tpm_default_chip()

When crypto agility support will be added to the TPM driver, users of the
driver have to retrieve the allocated banks from chip->allocated_banks and
use this information to prepare the array of tpm_digest structures to be
passed to tpm_pcr_extend().

This patch retrieves a tpm_chip pointer from tpm_default_chip() so that the
pointer can be used to prepare the array of tpm_digest structures.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>